1. Field of the Invention
The present invention relates to an image processing apparatus which performs image processing on image data obtained by utilizing a single-plate solid image pickup device, an image processing method and a medium where an image processing control program is stored.
2. Description of the Prior Art
A digital still camera or the like using a solid image pickup device usually employs a single-plate method. As shown in FIG. 23(a), in the single-plate method, color filters of R (red), G (green) and B (blue) are arranged in mosaic at predetermined ratios, in correspondence with respective pixels of the solid image pickup device. Especially, G color filters are zigzag arranged, in a high color component ratio. At each pixel of the solid image pickup device, color signals of the respective R, G and B colors cannot be obtained, but only one of R, G and B color signals is obtained. Accordingly, color signals that cannot be directly obtained at each pixel are obtained by interpolation from color signals of adjacent pixels, then all the R, G and B color signals are converted into multilevel data. The multilevel data are outputted, and display is made based on the multilevel data on a display device or the like.
For example, in FIG. 23(a), in the intermediate color filter array indicated by an arrow (→), the left half part from the center is irradiated with light (white part), while the right half part from the center is not irradiated with light (hatched part), as shown in FIG. 23(b). Assuming that the color signal level of respective colors in irradiated state is “1” and that of the same color signals in unirradiated state is “0”, the levels of the R and G color signals will have values as shown in FIG. 23(c).
However, as described above, a G color signal cannot be directly obtained from an R color filter, and an R color signal cannot be directly obtained from a G color filter. Accordingly, the G color signal corresponding to the R color filter is obtained from linear interpolation on the G color signal at an adjacent pixel. On the other hand, the R color signal corresponding to the G color filter is obtained from linear interpolation on the R color signal at an adjacent pixel. Then, the G and R color signal levels have the values as shown in FIGS. 23(d) and 23(e). As it is apparent from these figures, false color signals occur at pixels around the boundary between an irradiated area and an unirradiated area, and a color blur occurs on an image by this false color signal. Especially, the color blur remarkably appears on the boundary between gray and white color areas. Conventionally, to reduce such color blur, a smoothing filter (low-pass filter) is applied to color difference data of all the pixels constituting image data so as to make the color blur inconspicuous.
The conventional technique has the following problem.
The application of the smoothing filter corresponds to diffusion of one color component of one pixel to peripheral pixels; that is, matrix calculation is performed by using, e.g., 5×5 pixel matrix having one pixel as the center. For example, in the 5×5 pixel matrix calculation, calculation is performed 5×5=25 times on one pixel. Accordingly, in case of calculation on all the pixels, the calculation amount is enormously 25×the number of pixels, which increases processing time.